Understanding the role of exchange and correlations in complex oxides under strain and oxide heterostructures

Abstract

The study of complex oxides and oxide heterostructures has dominated the field of experimental and theoretical condensed matter research for the better part of the last few decades. Powerful experimental techniques such as molecular beam epitaxy and pulsed laser deposition have made fabrication of oxide heterostructures with atomically sharp interfaces possible, whereas more and more sophisticated handling of exchange and correlations within first principles methods including density functional theory (DFT) supplemented with Hubbard U corrections and hybrid functionals, and beyond DFT techniques such as dynamical mean field theory (DMFT) have made understanding of such correlated oxides and oxide interfaces easier. The emergence of the high-mobility two-dimensional electron gas with fascinating properties such as giant photoconductance, large negative magnetoresistance, superconductivity, ferromagnetism, and the mysterious coexistence of the latter two have indeed caught the attention of condensed matter community at large. Similarly, strain tuning of oxides have generated considerable interest particularly after the recent discovery of piezoelectric methods of strain generation. Theoretical understanding and prediction of the possible exotic phases emerging in such complex oxides both under strain and in heterostructures will eventually lead to better design of device applications in this new emerging field of oxide electronics, along with possible discovery of exotic physics in condensed matter systems, which may be of wider significance! In this review, we briefly look at theoretical studies of novel phenomena in oxides under strain and oxide heterostructure, and try to understand the role of exchange and particularly correlation in giving rise to such exotic electronic states. This review though primarily focuses on the theoretical aspects on understanding the different mechanism of the phenomena of emergence of exotic phases, does provide a unique overview of the experimental literature as well, accompanied by the theoretical understanding such that relevant device applications can be envisaged.

Stability and Redox Mechanisms of Ni-Rich NMC Cathodes: Insights from First-Principles Many-Body Calculations

“…Moreover, HSE06 is a nonlocal functional and is more apt, particularly for describing nonlocal states like O p states. , Although in recent times, the determination of Hubbard U for O 2p orbitals using mostly linear response methods have been carried out, and in some cases, these have yielded large values of Hubbard U indicating a localized behavior, this remains doubtful since such U values have only been applied in codes wherein those U calculations have been done leading to a loss of generality. In general, O 2p, in the context of the Hubbard model solved using DMFT, have not been considered to be strongly correlated, − ,,,, even in so-called dp model calculations . Thus, we conclude that calculations of observable properties should be performed at a level of theory that can correctly reproduce the insulating ground state.…”

“…For our DMFT calculations, we used U values ranging from 5 to 7 eV and J H = 0.5–0.75 eV depending on the TM ion in question. A choice of U of 6–7 eV for Ni, ,, 4–5 eV for Mn, ,,, and 5–6 eV , for Co sites were made based on the previous literature. Calculations were carried out for beta = 20 eV –1 , i.e., T = 580 K. We obtained real-frequency spectra using the maximum-entropy method of analytic continuation of imaginary time Green’s functions as implemented in the triqs/maxent application .…”